Generally speaking a touch-sensing system, including a digitizer, writing panel, etc, consists of two major parts, namely the touch-sensing tablet and the touch-input devices. The touch-input devices, such as a pen, a stylus, a human finger, a mouse, etc, are used to introduce the touch events. The touch-sensing surface is used to sense the relative position of the touch-input devices on its surface.
A touch-input device can be classified as an active device or a passive device according to whether it emits an excitation signal for the touch-sensing tablet to detect its position. For an active device that emits an excitation signal, a circuit to generate this signal and a method of supplying power to this circuit are needed.
Typical methods in current touch-screen technologies to supply power to the touch-input devices include using a battery, using Electromagnetic (EM) induction, and Infrared induction, etc. To supply power to the touch-input device through EM induction, the touch-sensing tablet first generates an EM field across its sensing surface to establish an energy field. When a touch-input device is placed within this EM field, its couples the energy of this EM field through a coil antenna and feeds this energy into an electrical power supply for its active circuit.
In U.S. Pat. Nos. 5,693,914, 5,594,215, and other similar patents, power supply methods incorporating EM induction in touch systems used as digitizers are disclosed. The touch systems use a 2-D antenna array to introduce the EM field onto the touch-sensing surface. This 2-D array consists of a rectangular loop antennae placed with a given interval in both X and Y directions of the touch sensing surface. In some patent disclosures some power control methods are introduced so that the loop antennae that are closest to the touch-input device are given maximum power, so that the resulting EM field distributes maximum energy to the space near the touch-input device.
This antenna configuration has many drawbacks, one of them being that these loop antennae must be placed underneath the touch-sensing surface so that enough power of the energy field can be introduced to the device. This placement may pose a significant problem in a computer touch screen application, where combining the screen surface and the loop antenna array together is a difficult engineering problem. And it is impossible for this touch-sensing assembly to be built separately, as most touch-screen assemblies are manufactured currently, and integrated with the existing computer systems.
Another drawback is that, as multiple loop antennae are used to generate the EM field, EM fields from these loop antennae could interfere with each other, providing a resulting EM field with poor uniformity and poor directivity. When the touch-input device is expected to couple with this EM field using its coil antenna with strong directivity, the coupling result may be degraded.